The invention relates to an overload coupling or clutch having coaxially arranged coupling or clutch halves, at least one of which is axially displaceable and is held in form-fitting engagement with the other by means of a spring force, and having a face gear on their facing ends for effecting the form-fitting engagement, wherein the flanks of the gear-tooth system that rest on one another during transmission of a torque generate an axial force component acting in the direction of disengagement and a radial force component.
Overload couplings of this type, such as are known from German Offenlegungsschrift DE-OS No. 27 44 908, serve as safety devices for machines and systems, so as to limit the torque at a suitable point in the drive train to a value at which damage will not yet occur. The face gears are pressed against one another by a predetermined, optionally adjustable axial force, and the transmitted torque, in accordance with the angle of engagement of the face gear, generates the axial force component which tends to drive the gear-tooth system apart. Therefore, as soon as the torque-dependent force exceeds the predetermined maximum force, the displaceable half of the coupling moves axially until such time as the gear-tooth system becomes disengaged and the coupling rotates freely or shuts off. However, the invention is also applicable to known coupling structures, operating in the same way, in which the teeth of one coupling half, or transmission elements meshing in a form-fitting manner with the gear-tooth systems of both coupling halves, are embodied as roller bodies.
An overload coupling of the type mentioned at the outset above is known from Swiss Pat. No. 237 460. It differs from the above-mentioned structure in that the flanks of the face gear are embodied as helical surfaces, which also encompasses arrangements of the helical surfaces which in addition to an axial force component also generate the above-mentioned radial force component. Embodying the flanks as helical surfaces serves to overcome the situation, which is otherwise present in overload couplings of the type under discussion here, in which the linear or areal contact of adjacent flanks that prevails when the coupling is engaged is lost, except for a point or edge contact, during the disengagement of the coupling, which is just the time when the load is at a maximum; the reason for this is that because of the obliquity of the flanks, the axial movement of the shiftable coupling half also involves a rotational movement, which causes associated flanks to be pivoted against one another.
A common feature of all the overload couplings described in the prior art is that the shiftable coupling half is rotationally connected in an axially displaceable way with the structural parts that support or adjoin it; this can be achieved by means of a serration, for instance, which will be the term used hereinafter for the sake of simplicity, but this term is understood to imply no restriction as to other structural forms of axially displaceable rotational connections. Solely in terms of manufacturing engineering, this serration dictates the shiftable coupling half will have play with respect to the structural part supporting it, especially when the coupling is in the engaged condition, so that the degree of angular accuracy in the rotational movement transmitted by the coupling that is needed in order to meet exacting requirements is impossible to attain. This angular accuracy is becoming increasingly significant, however, for instance in drive mechanisms with stepping motors for numerically controlled machines.